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STM8A Open Drain Pin Protection. Please Verify My Design

Question asked by berggren.seth on Dec 11, 2012
Hello, attached is a picture of the power input of my circuit and the protection diodes I have implemented. This is an automotive device with a 12V nominal input, that will get voltage spikes of up to 60V during engine ignition. I am using STM8AF52A8.

Looking at the schematic I attached, the input voltage from the car battery is on the left side of the fuse. On the right side of the fuse is the 12V input to the voltage regulator (not shown on schematic).

To protect the regulator, I have implemented a series of 2 TVS diodes. The bottom diode is rated for 3.3V reverse standoff, and the top diode is 18V reverse standoff. In between these diodes is a 250 Ohm resistor input to the STM8A mcu (port pin PE1). 

The purpose of this circuit is to detect the high voltage caused by engine ignition on Port Pin PE1 of the mcu.

From the STM8A datasheet, the absolute maximum voltage on PE1 is 6.5V. And the maximum injection current is 10mA. The datasheet also mentioned that, as long as the voltage at PE1 is kept under 6.5V, then the requirements for maximum injection current are inherently met. So, based on these values, I assumed the resistance at this pin to be 650 Ohms (6.5 / .01 = 650).

Based on the bottom TVS diode (D3) datasheet, I expect a voltage of between 4.5V and 7.5V at the junction between the 2 diodes and 250 Ohm resistor. A simple voltage divider equation for the 250 Ohm resistor, 650 Ohm pin, and input voltage of 4.5V to 7.5V, yields a voltage of between 3.25V and 5.42V at PE1. This should be adequate for causing an interrupt on the pin without destroying the mcu. Do these calculations seem correct?

Thank You